So here is what you need.
1: A vector image to screen print.
2: A blank screen
3: A sign squeegee, or credit card.
4: A vinyl decal or access to a vinyl cutter.
5: Masking/transfer tape.
Step 1: The image to decal
1: Find the image you want to screen print, and if necessary convert to a vector image.
(It will need to be vector due to the vinyl cutting stage only working with vectored images.)
2: Cut the image, or get it cut for you!
(its very important that you ask for it to be a negative of the image, or if your weeding the vinyl yourself you need to weed the line-work from the vinyl)